1. Field of the Invention
The present invention relates to overhead projectors.
2. Description of the Prior Art
Conventional overhead projectors of the transmissive type include a projector body having a condensing lens system defining an object stage and a light source located below the stage and within the body of the projector. A projection lens and mirror are supported above the stage by a projection head, with the projection lens interposed between the mirror and the stage. A slide or transparency is placed upon the stage and light from the light source is directed through the transparency and condensed to the projection lens. After exiting the projection lens, the light is reflected by the mirror to an imaging surface, usually a wall or a screen, to produce a magnified image of the transparency. The overhead projector is most useful for projecting magnified images of A-4 sized originals for group viewing.
The projection lens is usually parallel to the stage and overall focus of the image may be adjusted by varying the back focus of the projection lens, back focus being the distance of the projection lens from the object. If the mirror is oriented at 45 degrees, so that the reflected plane of the image is parallel to the plane of the projection lens and the plane of the object, the image will be uniformly in focus and undistorted because the Scheimpflug condition is met. The Scheimpflug condition dictates that the extended planes of the reflected image, projection lens and object must meet at a point or infinity for the image to be uniformly in focus.
It is sometimes necessary, however, for the mirror to be tilted to an angle above 45 degrees, relative to the plane of the object, in order to raise the image for greater viewing ease. In this situation, the Scheimpflug condition is no longer met since the reflected plane of the image is not parallel to the planes of the projection lens and the object, and the image is not uniformly in focus (Scheimpflug distortion). Also, the overall focus of the image is incorrect because the distance from the projection lens to the image (front focus) has increased. To correct the Scheimpflug distortion, it is necessary to tilt the projection lens in the direction of increased mirror tilt, and to correct the overall focus it is necessary to adjust the distance between the projection lens and the object (back focus) by translating the projection lens toward the object.
West German Offenlegungsschriften No. 3,136,970 addresses the problem of Scheimpflug distortion introduced when the image is raised by increased tilt of the reflecting mirror. The West German document discloses a projection head which includes a projection lens pivoted at its optic axis and a linkage connecting the mirror and the projection lens. As the mirror is rotated relative to the housing to change the location of the image, the linkage causes the projection lens to rotate in a direction corresponding to the rotation of the mirror. This coordinated movement of the mirror and the projection lens will correct for Scheimpflug distortion, assuming that the ratio of rotation between the projection lens and the mirror is appropriate, but will do nothing to correct for the loss of overall focus due to the increased front focus distance from the projection lens to the image. To obtain uniform focus, the entire projection head, including the mirror and the projection lens, will have to be adjusted to obtain the correct distance between the projection lens and the object.